This invention relates to a motorized power steering apparatus for automobiles or other vehicles. In the past, power steering apparatuses for automobiles were usually driven by hydraulic power generated by a pump. However, as hydraulic systems are bulky and heavy, in recent years, there has been a trend towards the use of electric motors to provide the drive force for power steering. A power steering apparatus which employs an electric motor to generate an auxiliary torque to assist the steering of the vehicle is referred to as a motorized power steering apparatus.
In a motorized power steering apparatus, a torque sensor measures the steering-torque applied by the driver to a steering wheel. An electric motor which is connected to a suitable portion of the steering gear is then controlled by a control unit in accordance with the measured torque to impart an auxiliary steering force to the steering gear. The auxiliary steering force lessens the force which need by applied to the steering wheel be the driver.
The control unit for controlling the motor typically employs semiconductor power switching elements. The semiconductor power switching elements and the motor are usually designed for operation for only a short period of time at a relatively low rated current. However, when the vehicle is stationary and the steering wheel is turned by the driver as far as it will go in one direction and maintained in that position for a long period, the steering torque sensor senses a large steering torque, and the control unit sends a large current through the motor for an extended length of time. As the control unit and the motor are not designed to handle a large continuous current for an extended period, there is a possibility of their overheating and burning out or being otherwise damaged by heat.
It is possible to design the motor and control unit to handle a large continuous current, but doing so makes them undesirably large and expensive.